1. Field of Invention
The present invention relates to fabrication of a semiconductor device. More particularly, the present invention relates to a method of fabricating a metal-oxide-semiconductor transistor (MOS transistor).
2. Description of Related Art
As the integration of the semiconductor device is increased, the size of the device is gradually decreased according to the reduced design rule. However, the resistance of the circuit becomes higher and higher when the material of the wiring line remains the same while the size is reduced.
Some methods to overcome the high sheet resistance of the poly gate and the source/drain junction have been developed in prior technique. One such method is a self-aligned silicide (salicide, SAC) process. SAC forms a metal silicide layer on the surface of the poly gate and the source/drain region, such that the sheet resistance of the poly gate and the source/drain region is lowered.
However, in the deep sub-micron level of semiconductor process, the surface area of the poly gate and the source/drain region provided to form a salicide becomes quite small, which results in a narrow line-width effect in the fabrication of salicide. In order to increase the area for salicide formation, the spacer beside the gate is over-etched to lower the height of the spacer so as to expose an upper edge of the poly gate sidewall, such that the area for forming the salicide is increased.
Although the high sheet resistance is improved by this method, other problems are created. The longer the spacer is over-etched, the shorter the spacer is, and the width of the spacer is reduced. The reduced width of the spacer causes relevant problems, such as a short channel effect due to the reduced width of the lightly doped drain (LDD). Therefore, it is difficult to control the time for which the spacer is over-etched. In addition, since the substrate surface of the source/drain region is exposed during over-etching, the substrate is easily damaged because of prolongation of etching time, which damage leads to current leakage.